lib/parser.c: switch match_number() over to use match_strdup()

This simplifies the code.  No change in behavior.

Link: http://lkml.kernel.org/r/20180830194727.191555-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>